Data from: Climatic thresholds shape northern high-latitude fire regimes and imply vulnerability to future climate change

Boreal forests and arctic tundra cover 33% of global land surface and store an estimated 50% of total soil carbon. Because wildfire is a key driver of terrestrial carbon cycling, increasing fire activity in these ecosystems would likely have global implications. To anticipate potential spatiotemporal variability in fire-regime shifts, we modeled the spatially explicit 30-yr probability of fire occurrence as a function of climate and landscape features (i.e., vegetation and topography) across Alaska. Boosted regression tree (BRT) models captured the spatial distribution of fire across boreal forest and tundra ecoregions (AUC from 0.63-0.78 and Pearson correlations between predicted and observed data from 0.54-0.71), highlighting summer temperature and annual moisture availability as the most influential controls of historical fire regimes. Modeled fire-climate relationships revealed distinct thresholds to fire occurrence, with a nonlinear increase in the probability of fire above an average July temperature of 13.4 °C and below an annual moisture availability (i.e., P-PET) of approximately 150 mm. To anticipate potential fire-regime responses to 21st-century climate change, we informed our BRTs with Coupled Model Intercomparison Project Phase 5 climate projections under the RCP 6.0 scenario. Based on these projected climatic changes alone (i.e., not accounting for potential changes in vegetation), our results suggest an increasing probability of wildfire in Alaskan boreal forest and tundra ecosystems, but of varying magnitude across space and throughout the 21st century. Regions with historically low flammability, including tundra and the forest-tundra boundary, are particularly vulnerable to climatically induced changes in fire activity, with up to a fourfold increase in the 30-yr probability of fire occurrence by 2100. Our results underscore the climatic potential for novel fire regimes to develop in these ecosystems, relative to the past 6,000-35,000 years, and spatial variability in the vulnerability of wildfire regimes and associated ecological processes to 21st-century climate change.

北方针叶林（Boreal Forest）与北极苔原（Arctic Tundra）占全球陆地总面积的33%，其储存的土壤碳约占全球土壤总碳量的50%。由于野火是陆地碳循环的关键驱动因子，这些生态系统野火活动的加剧或将产生全球性影响。为预测火状况变化的潜在时空变异，我们以阿拉斯加全域的气候与景观特征（即植被与地形）为自变量，构建了空间显式的野火发生30年概率模型。提升回归树（Boosted Regression Tree, BRT）模型可精准捕捉北方针叶林与苔原生态区的野火空间分布特征（受试者工作特征曲线下面积AUC为0.63~0.78，预测值与观测值间的皮尔逊相关系数为0.54~0.71），结果凸显夏季气温与年度水分可获得性为调控历史火状况的最关键因子。建模得到的火-气候关系揭示了野火发生的显著阈值：当7月平均气温高于13.4℃、年度水分可获得性（即降水减潜在蒸散量P-PET）低于约150mm时，野火发生概率呈非线性上升趋势。为预测21世纪气候变化下火状况的潜在响应，我们采用典型浓度路径RCP 6.0情景下的耦合模式比较计划第五阶段（CMIP5）气候预估数据驱动提升回归树模型。仅基于该气候预估结果（即未考虑植被的潜在变化），本研究结果显示阿拉斯加北方针叶林与苔原生态系统的野火发生概率将呈上升态势，但该趋势的强度在空间分布上以及整个21世纪内均存在差异。历史上可燃性较低的区域（包括苔原与林苔原交错带）对气候驱动的野火活动变化尤为敏感，至2100年时，这些区域的野火30年发生概率最高可提升至原水平的4倍。相较于过去6000~35000年的基准状态，本研究结果凸显了这些生态系统中出现新型火状况的气候可能性，同时也揭示了野火状况及相关生态过程对21世纪气候变化的脆弱性存在空间异质性。